This invention relates to a method of driving a matrix display device comprising sets of row and column address conductors, a row and column array of electro-optic display elements operable to produce a display, each of which is connected in series with a two terminal non-linear device between a row conductor and a column conductor, in which each row of display elements is driven by applying during a respective row address period a selection voltage signal to a row conductor to select the row of display elements and data voltage signals to the column conductors to drive each display element to produce a required display effect, in which, prior to the application of a selection voltage signal and a data voltage signal which are operable to charge a selected display element to a voltage of predetermined sign and magnitude at which the required display effect is obtained, the display element is charged to an auxiliary voltage of the same sign and greater magnitude. The invention relates also to a matrix display device drivable by such a method.
The display device may be used to display alpha-numeric or video information and the two terminal non-linear devices can be of various forms, such as diode rings, back to back diodes, MIMs, etc. which are bidirectional and substantially symmetrical. The display elements, for example, liquid crystal display elements, are addressed by sequentially applying a selection voltage signals to each one of the first set of address conductors in turn and applying in synchronised manner data signals to the other set as appropriate to drive the display elements to a desired display condition which is subsequently maintained until they are again selected in a following field period.
A method of driving a display device of the above kind is described in U.S. Pat. No. 5,159,325. In this method a five level row scanning signal is employed which includes a reset voltage signal in addition to the usual selection signals and non-selection (hold) levels. The selection and hold levels are polarity inverted for successive fields and, together with the reset voltage signal, which may be regarded as an additional selection signal, require a five level signal waveform. Before presenting a selection signal which together with the data signals provides the display elements of a row with a voltage of a certain sign, the display elements are charged through their non-linear devices having an approximately symmetrical I-V characteristic to an auxiliary voltage level of the same sign and which lies at or beyond the range of voltage levels (Vth to Vsat) used for display. During the application of the reset voltage the voltage applied to the column conductors may be set to zero volts. This method leads to a reduction of non-uniformities (grey variations) in the transmission characteristics of display elements which can otherwise occur when driving the rows with periodical inversion of the polarity of both the selection and the non-selection signals, simultaneously with inversion of the data signals. As described in that specification, the applied drive voltages can be arranged such that during a number of successive selection signals in successive fields applied to a row of display elements, which can include selection signals which are not preceded by a reset voltage for charging the display elements to an auxiliary voltage level, the current through the associated non-linear devices during selection periods has the same direction.
The drive scheme of U.S. Pat. No. 5,159,325 helps to compensate for the effects of non-uniformities in the operating characteristics of the non-linear devices of the display device.
Ideally, the non-linear devices of the display device should demonstrate substantially similar threshold and I-V characteristics so that the same drive voltages applied to any display element in the array produce substantially identical visual results. Differences in the thresholds, or turn-on points, of the non-linear devices can appear directly across the electro-optical material producing different display effects from display elements addressed with the same drive voltages. Serious problems can arise if the operational characteristics of the non-linear devices drift over a period of time through ageing effects causing changes in the threshold levels. The voltage appearing across the electro-optic material depends on the on-current of the non-linear device. If the on-current changes during the life of the display device then the voltage across the electro-optic material also changes. This change may either be in the peak to peak amplitude of the voltage or in the mean d.c. voltage depending on the actual drive scheme. The consequential change in display element voltages not only leads to inferior display quality but can cause an image storage problem and also degradation of the LC material.
In European Patent Specification EP-A-0523797 there is described a similar display device which further includes a reference circuit which comprises a capacitor connected in series with a non-linear device like those of the display elements and to which is applied drive signals similar to those applied to the display elements. Changes in the way in which the non-linear device of the reference circuit behaves reflect behavioural changes in the non-linear devices of the display elements and by monitoring the characteristics of the non-linear device of the reference circuit, correction can be made so as to compensate for the corresponding changes in the on-current of the display element non-linear devices due to ageing processes. To this end, a reference voltage is applied to the reference circuit simulating a data signal which corresponds to a predetermined average data signal level or is derived from actual data signals applied to column conductors over a period of time.
The effects of ageing of many non-linear devices, for example silicon nitride MIMs, are dependent to a large extent on the manner in which the device is operated. Changes in the device's operating characteristics are determined by the voltage levels to which the display element is driven. Driving a display element to higher values causes larger currents to flow through the non-linear device with the result that the rate of ageing is increased. The scheme described in EP-A-0523797 for correcting drift in the non-linear devices can compensate for the ageing of the non-linear devices driven to a single drive level. In practice, however, the ageing of the non-linear devices associated with picture elements which, in the case of LC display elements, are driven fully on (non-transmissive) and fully off (transmissive), e.g. black and white respectively, can be significantly different. Because the non-linear device of the reference circuit is driven at an intermediate, i.e. average, level it ages at a rate intermediate between the two extremes.